It is known that organoboron compounds, particularly boron alkyl compounds, can be used to trigger free radical polymerizations at room temperature. The oxygen required for polymerization is always present and does not have to be added separately. Until now, the preferred compounds have been simple trialkyl boron compounds, such as triethyl boron or tri-n-butyl boron. The use of such trialkyl boron compounds as polymerization initiators is described, for instance, in U.S. Pat. Nos. 3,476,727; 3,633,490; 2,985,633; and G.B. Pat. No. 1,113,722. U.S. Pat. No. 4,167,616 describes the use of the reaction products of butadiene and diborane as polymerization initiators.
Such boron alkyl compounds are also useful as hardeners for monomeric adhesives, particularly for (meth)acrylate adhesives. These adhesive systems contain, in addition to acrylic acid esters or methacrylic acid esters, trialkyl boron compounds, for example triethyl boron, tri-n-butyl boron, and the like as their essential component; see for example Japanese Pat. No. 4,214,318.
Trialkyl boron compounds of this type, however, have the well known and crucial disadvantage of igniting easily or even igniting spontaneously, perhaps even at room temperature, so that the handling of these adhesive mixtures creates significant difficulties. Attempts have been made to overcome this disadvantage by reacting the trialkyl boron compounds with 0.3 to 0.9 moles of oxygen; see German Patent Application 23 21 215. Additionally, attempts have been made to react the trialkyl boron compounds with amines in order to reduce their spontaneous ignition properties; see Japanese Pat. No. 4,529,195.
Although the above procedures shift the ignition temperature into the range of about 0.degree. to 70.degree. C., in actual practice the handling of these mixtures still presents a significant safety problem. Furthermore, the reactivity of these derivatives is greatly reduced.
It is generally known that when free boron alkyl compounds are exposed to oxygen they are rapidly oxidized to boric acid esters, which no longer act as polymerization initiators. Until now, it has therefore been necessary to eliminate any contact with oxygen when measuring out quantities of boron alkyls, and of course also during their manufacturing process, in order to preserve their activity as initiators. In fact, the required quantity of boron alkyl material must be packaged under inert gas into totally airtight storage vessels so that any possibility of oxygen entering the storage vessel is eliminated. The so-portioned boron alkyls must then be used quantitatively. Therefore, this basically interesting group of materials, namely boron alkyl compounds, and systems containing them as described in the prior art, are not suitable for practical use in adhesives.
However, there are significant advantages in the use of boron alkyl initiators; for example, that:
(a) polymerization reactions using them occur even at low temperatures, including room temperature; PA1 (b) the starter/hardener system is a single component system; and PA1 (c) the rate of polymerization can be controlled by varying the amount of oxygen available. PA1 R.sub.1 R.sub.2 R.sub.3 B, PA1 R.sub.1 R.sub.2 BH, and/or R.sub.1 BH.sub.2 ;
It is also known that certain selected boron alkyl compounds are more stable than others in the presence of atmospheric oxygen. These more stable compounds are usually boron compounds with sterically hindered alkyl radicals. Typical representatives of this class of compounds are diisopinocampheylborane, dicyclohexylborane, thexylborane(2,3-dimethyl-2-butylborane), 3,5-dimethylborinane, diisoamylborane, and particularly 9-borabicyclo[3.3.1]nonane, which is preferred due to practical considerations. Selected compounds of this type generally do not exhibit spontaneous ignition when they come in contact with atmospheric oxygen. On the other hand, under such conditions they quickly lose their ability to initiate polymerization, so that their use in actual technical processes involves significant difficulties. Additionally, these more stable boron alkyls dissolve with great difficulty or at least very slowly in many monomers, particularly in systems containing ester groups.